1. Field of the Invention
This invention relates to a high-frequency semiconductor technique, and more particularly, to a tape automated bonding tape or tape-like carrier (hereinafter referred to simply as "TAB tape") used to connect electrodes of a semiconductor device operating in a high frequency range, and circuit patterns formed on a ceramic board or printed board on which such a semiconductor device is mounted.
2. Description of the Related Art
A TAB tape comprises an insulation film made of, for example, a polyimide film strip having a number of longitudinal segments each provided with a plurality of conductive metalic circuit patterns formed of, for example, copper foil, electroplated with gold or the like. Such a TAB tape is more useful than a conventional wire-bonding, for a multi-pin semiconductor device.
Inner leads of the metalic patterns of the TAB tape are connected to respective electrodes or bumps made of, for example, gold or the like, and formed on the semiconductor chip, by an Au-Sn (gold-tin) eutectic alloy. Thus, a tin-plated film is usually formed on the respective lead patterns, made of copper foil, on the TAB tape, and such a tin-plated film is also formed on respective outer leads of the TAB tape, which are connected to outside electrical circuits.
Nevertheless,a known TAB tape as mentioned above has the following problems.
When the inner leads of the TAB tape are connected to the semiconductor chip by an Au-Sn (gold-tin) eutectic alloy, a high temperature bonding condition of 400.degree. C. to 500.degree. C. is generally required. Such a high temperature generated in the inner leads is immediately transmitted to the outer leads of the TAB tape, whereby the outer to the outer leads of the TAB tape, whereby the outer leads are heated, and thus the copper of the outer leads may be diffused into the tin-plated layer, and accordingly, the tin-plated layer is no longer effective. Therefore, the bonding efficiency between the outer leads and the outside electrical circuits would be adversely affected by such a diffusion of copper into the tin-plated layer.
Even if the outer leads are temporarily connected to the outside electrical circuits, the copper in the outer leads may be gradually diffused into the tin-plated layer in such a time since copper inherently has a characteristic of being easily diffused into tin. Therefore, the strength of the solder bonding may be weakened, and therefore, the soldered portions easily peeled off.
If a thickness of the tin-plated layer is increased, the above-mentioned problems may be overcome to a certain extent, but in this case, when the inner leads of the TAB tape are connected to the semiconductor chip by an Au-Sn (gold-tin) eutectic alloy, an excess amount of tin may flow to thereby cause a short circuit between adjacent inner leads. Therefore, the tin-plated layer must be thin, i.e., have a thickness thereof of approximately 0.5 .mu.m.
On the other hand, when the inner leads of the TAB tape are connected to the semiconductor chip, a bonding tool is used to urge the inner leads toward the bumps of the semiconductor chip, and as a melting solder of tin may be attached to the bonding tool the bonding tool must be frequently cleaned. This can lead to a state, wherein such a bonding tool becomes no longer useful as a bonding tool.
In addition, the tin-plated film is readily deteriorated over a long time, due to, for example, an oxidization of the surface of the tin-plated film. Therefore, when the inner leads of the TAB tape are connected to the semiconductor chip by an Au-Sn (gold-tin) eutectic alloy, a favorable meniscus due to the gold-tin eutectic alloy can not be obtained. Also, the tin-plated film develops very thin needle-like whiskers or the like, which sometimes cause a short circuit between the densely arranged leads of the conductive circuit patterns.